1. Field of the Invention
The present invention relates generally to increasing the residence time of reactants in a reactor. More particularly, the present invention relates to an apparatus and method for increasing the gas residence time in a chemical vapor deposition reactor.
2. Description of the Invention Background
Chemical vapor deposition (CVD) is known as a technique for forming solid films on a substrate by the reaction of vapor phase chemicals near and preferably on the surface of the substrate to produce a solid film. CVD techniques have been particularly useful in the microelectronics industry because CVD techniques can be used to reliably produce extremely thin films, or layers, having good coverage characteristics and structural uniformity. In the production of electronic devices, CVD techniques are used to selectively deposit layers on a large wafer-shaped silicon substrate, or wafer, to form a plurality of complex electronic circuit elements separated by narrow streets. The wafers are then cut along the streets to separate the individual elements into chips, or dies, and leads are attached to form the electronic devices.
In general, CVD techniques involve the delivery of gaseous reactants to the surface of a substrate where chemical reactions take place under temperature and pressure conditions that are favorable to the thermodynamics of the desired reaction. The temperature of the reaction can be controlled by either controlling the temperature of the entire reactor, as in a hot-wall reactor, or by locally controlling the temperature of the substrate as in a cold-wall reactor. The energy supplied to the reactor to control the temperature can be entirely thermal, or can be partially plasma or photon induced.
The system pressure of the reactor is significant in controlling not only the rate of the chemical reactions, but the characteristics of the deposited film. Most early CVD reactors were atmospheric pressure CVD (APCVD) reactors, which provide for high deposition rates due to the relatively high concentration of the reactants and the high reaction rates that can be achieved at atmospheric pressures. However, many of the chemical reactions used to form the films occur at a sufficiently high rate at APCVD conditions that the transport of reactants to the surface is the rate limiting step in the reaction. In mass transport controlled reactions, the concentration of the reactants must be precisely controlled and uniformly maintained throughout the reactor in order to produce films having uniform thicknesses and good coverage characteristics. Another persistent problem in APCVD reactors is the thermodynamic favorability of homogeneous gas phase reactions that produce particulate contamination that often results in nonuniform, friable films on the substrate.
Low pressure CVD (LPCVD) reactors typically operate at pressures less than 100 torr and provide a solution to some of the problems encountered with APCVD reactors. In LPCVD reactors, the lower pressure results in increased reactant diffusivities and decreased reaction rates, which in some cases is sufficient to change the rate controlling step of the reaction from mass transport to the surface to the reaction rate at the surface. The formation of the film under reaction rate limited conditions is beneficial from the standpoint that reactors can be designed to process a large number of substrates at one time, because a uniform reactant concentrations are readily achievable in these systems, which in turn increases the probability of producing a uniform thickness film. In addition, LPCVD greatly reduces the extent of homogeneous gas phase reactions, which reduces the amount of particulate contamination of the film, resulting in a more structurally uniform film having greater step coverage and generally more desirable surface characteristics than produced by APCVD. One disadvantage of LPCVD is that the reaction rates are significantly lower than with APCVD at the same temperature; therefore, in order to produce films of the same thickness as produced using APCVD in LPCVD reactors, either the processing time will have to be increased and/or the LPCVD reactors must be operated at higher temperatures to increase the reaction rates. Alternatively, the partial pressures of the reactants can be increased to increase the reaction rate; however, increasing partial pressures of the reactants tends to increase the homogeneous gas phase reaction rates that tend to degrade the film characteristics.
Another complicating factor is the effect of flow rate on the formation and characteristics of the film. The extent of reaction, and the corresponding reactant concentration, is dependent upon the residence time of the reactants in the reactor, especially under mass transport rate controlled conditions. In many reactors, recirculation cells are formed during operation trapping reactants with the recirculation cell, such that the trapped reactants have a different residence time than gaseous reactants in other parts of the reactor. The variation in residence time results in nonuniform film formation due to gas phase compositional differences across the surface of the substrate. One possible solution to the problems associated with the formation of recirculation cells near the surface of the substrate is presented in U.S. Pat. No. 4,976,996 issued to Monkowski. The Monkowski patent provides for laminar parallel flow over the surface of a substrate using a cylindrical annular reactor in which the gaseous reactants are introduced into the reactor through the periphery and directed parallel to the surface of the substrate by flow controllers, such as straightening vanes. The gas is further prevented from recirculating through the use of a top portion that is directly opposite and parallel to the surface of the substrate and through the additional flow controllers at the annular exit of the reactor. While the apparatus of the Monkowski patent provides for a predictable flow pattern in the reactor, the constraints on the reactor design make it difficult to modify for processing large numbers of substrates at one time. Also, because the flow of gas is laminar and parallel to the surface of the substrate, the access of reactants to the surface substrate is limited to diffusion across the streamlines of the flow resulting in stratification of the reactants and, as a consequence, deposition rates may be undesirably decreased in the presence of parallel laminar flow.
The present invention is directed to apparatus for increasing the gas residence time of reactants in a CVD reactor which overcomes, among others, the above-discussed problems so as to provide a more efficient method of depositing uniform layers on substrate using the CVD techniques.